"The mantle of the earth is made mostly of a mineral called olivine, and the assumption is usually that all planets are like the Earth," said Jay Melosh, a professor of earth, atmospheric and planetary sciences at Purdue University, who led the study. "But when we look at the spectral signature of rocks exposed deep below the moon's surface, we don't see olivine; we see orthopyroxene."

Around 4 billion years ago, an asteroid collided with the moon and created the largest and deepest impact on the moon: the South Pole-Aitken basin. The collision exposed lunar mantle in the basin and splashed up material onto the far side of the moon.

Melosh's group used remote sensing to identify what minerals compose the splashed-up material. When sunlight hits the moon, it interacts with materials on the surface; because different materials absorb different wavelengths of light, researchers can tell what materials are on the surface by looking at the reflected signal.